Vibrating crystal transducers are frequently used in modern sensors to provide an electrical signal representative of temperature. Vibrating crystals may be formed out of piezoelectric material. When a voltage is applied, this material stresses such that the shape of the material is deformed and, conversely, when a stress is developed on the material, a voltage develops on the surface of the material. A vibrating crystal has a frequency, called resonant frequency, at which the cyclical stressing results in a peak admittance, the minimal impedance, to the applied voltage. In some crystals, the resonant frequency is not constant, but varies with respect to the temperature of the crystal. Temperature sensors that employ vibrating crystal transducers operate by applying a voltage to the crystal to establish its resonant frequency which, in turn, is measured as an indication of the temperature of the crystal and the surrounding environment.
Many vibrating crystal transducers are in the shape of a miniaturized tuning fork with two tines that are connected together at a common base which is firmly secured to a sensor frame. Electrodes on the tines and/or the base portion of the crystal provide the necessary phase and amplitude of voltage to cause the tines to vibrate at a stable amplitude. Vibrating crystal transducers used in temperature sensors are usually arranged to vibrate in the torsional mode, wherein the movement of each tine is a rotation, or twist, about the longitudinal axis of the tine. When such a vibrating crystal transducer is electrically excited, the tines vibrate symmetrically with respect to each other. In other words, one tine rotates clockwise and the other rotates counterclockwise. In a subsequent phase of vibration, the orientations of the rotation of the individual tines are reversed.
Torsionally vibrating crystal transducers have large temperature coefficients that make it possible for the transducers to provide extremely accurate temperature measurements. Torsionally vibrating crystal transducers also vibrate at high frequencies, around 250 KHz, in comparison to other vibrating crystals, for instance, flexurally vibrating crystals that vibrate at approximately 35 KHz. The high resonant frequencies of torsionally vibrating crystals are sharply defined, which simplifies determining the exact instantaneous resonant frequency of the crystals. Thus, for many purposes, where extremely accurate temperature measurements are required, torsionally vibrating crystal transducers are a preferred type of temperature sensor.
Moreover, torsionally vibrating crystal transducers are basically insensitive to acceleration. This feature makes the crystals readily suited for placement in vehicles and other moving objects where it is desirable to measure environmental temperature, since there is little possibility the crystals will be affected by the movement of the object. For example, torsionally vibrating crystals are well suited for incorporation in acceleration units where the temperature measurement they provide is used to compensate for temperature variation in vibrating crystals used to measure acceleration.
A disadvantage of the conventional tuning fork type crystal temperature transducers is that the energy of the torsional motion of the individual tines about parallel but not co-linear axes is transferred directly to the base in the form of an antisymmetric pair of bending moments. The bending of the base results in a loss of the energy applied to excite the tines to vibrate. Consequently, relatively high voltages must be applied to the crystal in order to sustain resonant vibration of the tines. The bending of the base, and consequent loss of energy, lowers the overall quality factor of the crystal, which is a measure of sharpness or narrowness of the range of frequencies at which, at a given instant, the crystal may resonate. A crystal that, at a given instant, has only a very narrow, sharply defined resonant frequency has a high quality factor. A crystal with a broader, less defined resonant frequency is said to have a low quality factor. A low quality factor results in a less precise measurement of temperature. Moreover, a crystal with a low quality factor is more easily distrubed by environmental factors other than temperature.
Another disadvantage of a conventional tuning fork crystal is that it is sensitive to the mounting conditions at the point where its base is attached to the sensor frame. This sensitivity means that variations in mountings cause the quality factor of individual crystals to differ. These individual variations in crystal quality factor increase the difficulty of optimizing the oscillator electronics when the crystals are installed in temperature sensor assemblies.
Furthermore, the vibrational energy transferred to the base of the crystal is transferred through a sensor frame to adjacent transducers, for example, adjacent vibrating crystal transducers used to measure force in accelerometers. The energy transferred to the adjacent transducers can adversely affect the ability of these transducers to accurately provide signals representative of the parameters they are designed to monitor. Still another problem associated with vibrational energy transferred to the base is that, over time, the induced bending can significantly weaken the bond between the crystal and the sensor frame. The weakening of this bond can cause the resonant frequency of the crystal to change radically, and can even cause the crystal to break free of the body. In either case, the loosening of the crystal-to-sensor frame bond will render the transducer virtually useless for the purpose of making temperature measurements.
Moreover, since the crystal-to-sensor frame bond is so delicate, and because the crystal itself is so small, a temperature sensor assembly that includes a torsionally vibrating crystal is often very fragile. This has made it difficult to use a sensor with a torsionally vibrating crystal temperature transducer in an environment where the sensor would be subjected to significant mechanical shock.
An alternative to providing traditional tuning fork type vibrating crystals has been to provide beam-shaped crystals arranged so that the opposed half-sections of the beam vibrate torsionally with equal and opposite moments. The vibrational forces cancel out along a nodal line located at the center of the beam. Consequently, along the nodal line the beam does not vibrate, and therefore, energy is neither removed from nor taken out of the beam through the nodal line. A limitation associated with these crystals is that their vibration-free areas are quite narrow, merely the width of their nodal lines. In order to secure most vibrating beam crystals in place, these beams are often attached to the adjacent sensor frame along a line significantly wider than the width of a nodal line. Thus, even though beam-shaped vibrating crystal transducers are useful for eliminating some of the undesirable energy transfer to and from the transducer, they are not capable of blocking all of the energy transfer.